Current systems of time division multiplexing passive optical network (TDM PON), such as non-limiting examples of GPON, XGPON1, EPON, and 10G EPON, can be categorized as sharing wavelength architectures. In TDM, each transmitter transmits during a slice of the transmission period. An optical network unit (ONU) is a device that transforms incoming optical signals into electronics at a customer's premises in order to provide telecommunications services over an optical fiber network. In TDM PON, multiple ONUs share the wavelength, or the bandwidth of a particular wavelength in TDM fashion.
In order to provide higher bandwidth per ONU, next generation PONs explore the bandwidth of a fiber, i.e. utilize the full spectrum of a fiber rather than an individual wavelength. One method to increase the bandwidth per ONU is to assign a dedicated wavelength to each ONU. Many wavelength division multiplexing (WDM) PON architectures are based on wavelength routers. Either thin film filter or arrayed waveguide grating (AWG) technologies may be used in WDM PON architectures. An arrayed waveguide grating permits a single optical fiber to carry multiple channels or communication bands. Fiber optic cables use very thin glass fibers to transmit light signals containing voice or data communications.
Light passes through air or fiber cables as a series of waves, similarly to waves in water. The principle of light diffraction, where light passing through fibers of slightly different lengths exits at slightly different phases or angles, is the basis for an arrayed waveguide grating. Light exits each of the fibers in the waveguide at a slightly different point in the wave because each fiber has a different length, and the light takes more or less time to travel its length. When these out-of-phase frequencies interact, they create a diffraction pattern, which is a series of evenly spaced light signals, each with its own frequency.
In WDM, different frequencies of the light signal are used for different communication bands, and the arrayed waveguide grating is used to combine or multiplex these individual bands into a single fiber cable, allowing for many conversations or data streams to be combined. The process can be reversed at the other end of a transmission line, with the combined signals separated in a de-multiplexing waveguide.
There are few parts to an arrayed waveguide grating. The incoming fiber cable is connected to a mixing zone, with multiple fiber cables. The arrayed waveguide is lined up in a row at the other end of the zone. At the opposing end is a collection or focusing zone where the different wavelengths or channels are separated by diffraction and enter multiple fiber cables.
FIG. 1 provides system diagram 100 of a WDM PON architecture using an array of multiple wavelength transmitters as light sources in an optical line termination (OLT) system. An AWG is used in the field to route wavelengths to the ONUs using the cyclic property of an AWG to route upstream light from ONUs back to the OLT. The array of either fixed or general wavelength transmit lasers send the different wavelengths on transmission line 140 and all the wavelengths are coupled to one fiber. Multi-wavelength transmitter array 110 is coupled to fiber 140 at coupler 130, transmitting each wavelength, λ1 to λN, on the fiber. The downstream transmission signals are received at AWG 150 and then split out to individual ONUs with transmit/receive pairs 160, 170 at customer premises. In this implementation, the upstream data streams are sent at a wavelength of λN+1, which is the receive wavelength+N wavelength steps. The signals are combined in AWG 150 to fiber 140 and received at the head end by receiver array 120. ONU 160 uses a cyclic property of AWG 150 to send λN+1, to the same port and the same filter on the same fiber. At the head end, there's a separate filter at the receiver. At the receiver array, there's another filter so the λN+1 gets separated to the appropriate receiver. So in this case, one ODN and one fiber handle both the upstream and the downstream transmissions.
Wavelength router based WDM PONs have many advantages. For instance, the passive AWG has much lower loss than an active power splitter and its loss is independent of the number of wavelengths. Additionally, WDM PON has the potential of supporting more ONUs than does GPON or EPON, etc. However, there are large numbers of GPON and EPON systems already deployed in the field that use passive optical power splitters. There are heretofore unaddressed needs to reuse power splitters for WDM PON with these previous solutions.